This invention relates to a Doppler tracking processor and time of closest approach detector for use in a semi-active or active continuous wave (CW) radar system. An example of that is the application of the invention to CW semi-active guided missiles. So, before presenting the invention, its context will be introduced in the following text.
With guided missiles used against moving targets, such as aircraft, it is necessary to detonate the missile at the time of closest approach (TCA) to the target because of the unlikelihood of the missile scoring a direct hit on the target. The problem consists in detecting accurately the time of closest approach (TCA) to the target by processing the Doppler signal in the seeker of a CW radar semi-active missile when the TCA detector has been armed by other missile subsystems, such as a safety-arming (S&A) device which maintains the missile warhead in an unarmed condition until the missile has been intentionally launched and has traveled a safe distance from the launching aircraft.
Guidance and control of missiles are based on different techniques of major importance (beam rider guidance, command guidance, preset guidance and homing guidance). One of them, the homing guidance, generates steering signals from information received at the missile seeker from the target.
Homing guidance systems are of three types: active, semi-active and passive systems. An active homing system beams a signal at the target and generates steering commands from the reflected signal. This homing device on the missile reveals the presence of the missile to the target. In a semi-active homing system, a remote transmitter, located outside the missile (on an airplane, a ship or any other equipment with an appropriate antenna to illuminate the scenario), bounces signals off the target to the missile. The remote illuminating transmitter reveals its position and reveals the existence of a missile to the target. A passive homing system receives radiated energy emanating from the target. The passive homing device does not reveal the position or the existence of the missile to the target by emitting any energy (as a target, a missile radiates energy...). In these three systems radiated energy may be radio, optical (IR, UV and visible light) and sound signals.
The realization of the present invention is concerned with a CW radar "semi-active" missile. Even if the missile has only a receiver, it is called "semi-active" since it requires an external illumination to guide on the target. The energy transmitted from the illuminator transmitter external to the missile is received by rear and front antennas on the missile. The rear signal is the sample of the radiated energy from the transmitter used as a reference. The front signal is the energy from the illuminator that has been reflected by the target. These microwave signals are mixed and low-pass filtered to deliver a signal at lower frequency. This signal is termed the video Doppler since it is not necessarily a narrow-band signal and because it includes relative missile-target information with noise and interference.
The target signal is the radio energy returned to a radar by a target, also known as echo signal or video signal. Video is pertaining to the demodulated radar received output that is applied to a radar indicator. In low-altitude environments, sea clutter is often dominant. With targets composed of several reflectors, forward scatter from reflectors already passed, and backscatter from reflectors yet to be passed are sometimes important contributors to the Doppler.
It is known to use a fractional Doppler gate (FDG) to process the Doppler signal to activate the warhead detonation properly at TCA. In that prior arrangement the TCA Doppler detection is accomplished by a tunable band-pass filter (constant bandwidth), adjusted at a fractional Doppler frequency, preceded by a fast automatic gain control unit to normalize the entire signal at its input, and followed by an envelope detector and a high-pass filter (differentiator) in such a way as to detect only amplitude increases greater than a fixed threshold of the signal at the output of the band-pass filter. This TCA Doppler detector indicates when the doppler energy has rolled off to a fractional frequency within the constant bandwidth of the band-pass. The time-frequency plane is related to the TCA in the time-distance plane, by the physical and the geometrical characterization of the intercept (this will be explained later). The FDG center frequency is tuned at about one-half the preintercept Doppler frequency, 0.5f.sub.do.
Assuming that all the other arming subsystems operate properly, the present invention subsystem achieves a better signal processing than by the FDG since:
1. It is not sensitive to amplitude variation, as it reacts as a frequency discriminator. PA0 2. It has to detect the Doppler frequency decrease from its pre-intercept value (f.sub.do) down to a fraction of f.sub.do which corresponds to the TCA. This avoids premature TCA detection.